135 research outputs found
DGM : Directed-Graph Mapping as a novel tool to analyze cardiac arrhythmias
No full textCardiac arrhythmias and related diseases are the leading cause of death in the Western world. The management thereof currently accounts for about 9\% of the total health-care expenditure across the EU. Research on the mechanisms of arrhythmia and optimizations of diagnostic tools remain important.
The normal rhythm of the heart results in about 60 to 100 beats per minute, which accumulate roughly to about 3 billion beats in a lifetime. It is likely for this rhythm to get disturbed once in a while. Naturally, the heart rate increases with exercise or slows down during sleep. Drugs, alcohol, nicotine and the general health condition of a person may also affect the heart's condition. However, persistent abnormality of the heart rhythm can indicate cardiac arrhythmia and the associated complications will depend on the type of arrhythmia. In general, complications and symptoms of cardiac arrhythmias may include a racing or slow heartbeat, chest pain, shortness of breath or even anxiety and fatigue. In more severe cases cardiac arrhythmia can induce stroke, heart failure and sudden cardiac death.
Treatment of arrhythmia may include medication, specific therapies such as vagal maneuvers and cardioversion, surgery like ablation and maze procedures or the placement of implantable devices lika a pacemaker or defibrillator.
For this work, mainly ablation therapy is of interest. During ablation the electrophysiologist (EP) inserts a catheter (a measuring device with electrodes) into the heart and records the electrical activity. These electrical signals are presented on a color-coded map. Based on an educated interpretation of these color maps and corresponding signals the EP will gather knowledge about the mechanism maintaining the arrhythmia. Once the mechanism is understood and located, the EP will ablate or scar the heart tissue in order to stop electrical propagation in that specific region. This will stop the arrhythmia restoring the normal heart rhythm or convert the excitation pattern to a (usually) slower arrhythmia.
It can be very challenging to determine the mechanism of an arrhythmia and wrong ablation lines can make the heart prone to new arrhythmias and limits the myocardial contractility. Analysis of these electrical maps can be difficult and prone interpretation. Therefore, there is a need for automated and operator independent interpretations and strategies.
My dissertation is devoted to the development, applicability and accuracy of Directed-Graph Mapping or DGM on analysis of cardiac arrhythmia mechanisms. DGM is a mathematical approach based on concepts of network theory describing the properties of cardiac excitation waves. DGM takes as input the spatial coordinates of the electrodes and the Local Activation Time (LAT) of the signals. With this data, a directed graph is created. Based on validated algorithms of graph theory and new algorithms I designed, DGM automatically analyzes these graphs and presents the rotational circuit or focal sources of the arrhythmia under study
A study of the Gribov-Zwanziger framework: from propagators to glueballs
No full textThis thesis presents a study of the Gribov-Zwanziger framework: from propagators to glueballs. The chapters 2 and 3 are meant as an introduction and only require a basic knowledge of quantum field theory. Chapter 2 explains the techniques behind algebraic renormalization, which shall be widely used throughout this thesis, while chapter 3 tries to give a pedagogic overview of the Gribov-Zwanziger framework as this is not available yet in the literature. The subsequent chapters contain own research. First in chapter 4, we shall dig a bit deeper in the Gribov-Zwanziger framework, by exploring the BRST symmetry and the KO criterium. Next, in chapter 5 we shall elaborate on the ghost and the gluon propagator in the infrared and present a refined Gribov-Zwanziger action. Further, we present two chapters on the search for physical operators within the (refined) Gribov-Zwanziger framework, chapter 6 and 7. A small chapter 8 is devoted to some values for different glueballs. We end this thesis with the conclusions, chapter 9
On the reanimation of a local BRST invariance in the (Refined) Gribov-Zwanziger formalism
No full textEvaluation of directed graph-mapping in complex atrial tachycardias
No full textOBJECTIVES Directed graph-mapping (DGM) is a novel operator-independent automatic tool that can be applied to the identification of the atrial tachycardia (AT) mechanism. In the present study, for the first time, DGM was applied in complex AT cases, and diagnostic accuracy was evaluated. BACKGROUND Catheter ablation of ATs still represents a challenge, as the identification of the correct mechanism can be difficult. New algorithms for high-density activation mapping (HDAM) render an easier acquisition of more detailed maps; however, understanding of the mechanism and, thus, identification of the ablation targets, especially in complex cases, remains strongly operator-dependent. METHODS HDAMs acquired with the latest algorithm (COHERENT version 7, Biosense Webster, Irvine, California) were interpreted offline by 4 expert electrophysiologists, and the acquired electrode recordings with corresponding local activation times (LATs) were analyzed by DGM (also offline). Entrainment maneuvers (EM) were performed to understand the correct mechanism, which was then confirmed by successful ablation (13 cases were centrifugal, 10 cases were localized re-entry, 22 cases were macro-re-entry, and 6 were double-loops). In total, 51 ATs were retrospectively analyzed. We compared the diagnoses made by DGM were compared with those of the experts and with additional EM results. RESULTS In total, 51 ATs were retrospectively analyzed. Experts diagnosed the correct AT mechanism and location in 33 cases versus DGM in 38 cases. Diagnostic accuracy varied according to different AT mechanisms. The 13 centrifugal activation patterns were always correctly identified by both methods; 2 of 10 localized reentries were identified by the experts, whereas DGM diagnosed 7 of 10. For the macro-re-entries, 12 of 22 were correctly identified using HDAM versus 13 of 22 for DGM. Finally, 6 of 6 double-loops were correctly identified by the experts, versus 5 of 6 for DGM. CONCLUSIONS Even in complex cases, DGM provides an automatic, fast, and operator-independent tool to identify the AT mechanism and location and could be a valuable addition to current mapping technologies. (J Am Coll Cardiol EP 2021;7:936-49) (c) 2021 The Authors. Published by Elsevier on behalf of the American College of Cardiology Foundation. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/)
No-pole condition in Landau gauge: Properties of the Gribov ghost form factor and a constraint on the 2d gluon propagator
No full textWe study general properties of the Landau-gauge Gribov ghost form factor sigma(p(2)) for SU(N-c) Yang-Mills theories in the d-dimensional case. We find a qualitatively different behavior for d = 3, 4 with respect to the d = 2 case. In particular, considering any (sufficiently regular) gluon propagator D(p(2)) and the one-loop-corrected ghost propagator, we prove in the 2d case that the function sigma(p(2)) blows up in the infrared limit p -> 0 as -D(0) ln(p(2)). Thus, for d = 2, the no-pole condition sigma(p(2)) < 1 (for p(2) > 0) can be satisfied only if the gluon propagator vanishes at zero momentum, that is, D(0) = 0. On the contrary, in d = 3 and 4, sigma(p(2)) is finite also if D(0) > 0. The same results are obtained by evaluating the ghost propagator G(p(2)) explicitly at one loop, using fitting forms for D(p(2)) that describe well the numerical data of the gluon propagator in two, three and four space-time dimensions in the SU(2) case. These evaluations also show that, if one considers the coupling constant g(2) as a free parameter, the ghost propagator admits a one-parameter family of behaviors (labeled by g(2)), in agreement with previous works by Boucaud et al. In this case the condition sigma(0) <= 1 implies g(2) <= g(c)(2), where g(c)(2) is a "critical" value. Moreover, a freelike ghost propagator in the infrared limit is obtained for any value of g(2) smaller than g(c)(2), while for g(2) = g(c)(2) one finds an infrared-enhanced ghost propagator. Finally, we analyze the Dyson-Schwinger equation for sigma(p(2)) and show that, for infrared-finite ghost-gluon vertices, one can bound the ghost form factor sigma(p(2)). Using these bounds we find again that only in the d = 2 case does one need to impose D(0) = 0 in order to satisfy the no-pole condition. The d = 2 result is also supported by an analysis of the Dyson-Schwinger equation using a spectral representation for the ghost propagator. Thus, if the no-pole condition is imposed, solving the d = 2 Dyson-Schwinger equations cannot lead to a massive behavior for the gluon propagator. These results apply to any Gribov copy inside the so-called first Gribov horizon; i.e., the 2d result D(0) = 0 is not affected by Gribov noise. These findings are also in agreement with lattice data.ResearchFoundation Flanders (FWO)Research-Foundation Flanders (FWO)Ghent University (BOF UGent)Ghent University (BOF UGent)CNPqCNPqFAPESPFAPES
Effect of myocyte-fibroblast coupling on the onset of pathological dynamics in a model of ventricular tissue
No full textManaging lethal cardiac arrhythmias is one of the biggest challenges in modern cardiology, and hence it is very important to understand the factors underlying such arrhythmias. While early afterdepolarizations (EAD) of cardiac cells is known to be one such arrhythmogenic factor, the mechanisms underlying the emergence of tissue level arrhythmias from cellular level EADs is not fully understood. Another known arrhythmogenic condition is fibrosis of cardiac tissue that occurs both due to aging and in many types of heart diseases. In this paper we describe the results of a systematic insilico study, using the TNNP model of human cardiac cells and MacCannell model for (myo) fibroblasts, on the possible effects of diffuse fibrosis on arrhythmias occurring via EADs. We find that depending on the resting potential of fibroblasts (VFR), M-F coupling can either increase or decrease the region of parameters showing EADs. Fibrosis increases the probability of occurrence of arrhythmias after a single focal stimulation and this effect increases with the strength of the M-F coupling. While in our simulations, arrhythmias occur due to fibrosis induced ectopic activity, we do not observe any specific fibrotic pattern that promotes the occurrence of these ectopic sources
Unitarity analysis of a non-Abelian gauge invariant action with a mass
No full textIn previous work done by us and coworkers, we have been able to construct a local, non-Abelian gauge invariant action with a mass parameter, based on the nonlocal gauge invariant mass dimension two operator F-mu nu(D-2)F--1(mu nu). The renormalizability of the resulting action was proven to all orders of perturbation theory, in the class of linear covariant gauges. We also discussed the perturbative equivalence of the model with ordinary massless Yang-Mills gauge theories when the mass is identically zero. Furthermore, we pointed out the existence of a Becchi-Rouet-Stora-Tyutin (BRST) symmetry with corresponding nilpotent charge. In this paper, we study the issue of unitarity of this massive gauge model. First, we provide a short review how to discuss the unitarity making use of the BRST charge. Afterwards we make a detailed study of the most general version of our action, and we come to the conclusion that the model is not unitary, as we are unable to remove all the negative norm states from the physical spectrum in a consistent way
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